Method for the formation of a diffraction grating

ABSTRACT

A method for the formation of a diffraction grating on a substrate using a holographic technique and an etching technique, wherein the periodicity of the pattern of the diffraction grating can be changed at will by a change of the light-path length of one of the two light fluxes from a holographic exposing system.

This apllication is a continuation of application Ser. No. 812,784,filed Dec. 23, 1985, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for the formation of diffractiongratings. More particularly, it relates to a method for the formation ofdiffraction gratings with a periodicity of approximately 2000 Å on thesurface of a semiconductor substrate, in which the phase of thediffraction gratings can be changed at will.

2. Description of the Prior Art

A diffraction grating with a periodicity is formed on the surface of asemiconductor crystal, resulting in the reflecting surface of adistributed-feedback (DFB) laser. This distributed-feedback laser isadvantageous over other lasers in that cleavage of the crystal is notrequired for the formation of the resonator, integration can be readilyattained, and stabilized single-mode operation can be readily provided.However, as shown in FIG. 1(I), with a distributed-feedback laser inwhich the diffraction grating 3 composed of grooves 2 with a periodicityon the upper surface of substrate 1, stabilized oscillation in a singlemode cannot be achieved. In particular, it is not possible to obtainsingle-mode operation when high-speed modulation is involved, as whenthe laser is being used as a light source for optical communicationapparatus. In order to solve this problem, a distributed-feedbacksemiconductor laser such as that shown in FIG. 1(II) has been proposedin recent years in which the grooves 2' of the diffraction grating 3' inthe region of the center of the resonator is shifted by half a cycle(namely, the phase of the pattern of the diffraction grating is shiftedby π).

The phase shift of the diffraction grating in the middle of the laserresonator is achieved as follows: A photoresist is coated on asemiconductor substrate and exposed by an electronic beam-exposingsystem and then developed to form the portions corresponding to thegrooves of the diffraction grating. The substrate is then etched with anetchant. The remaining photoresist on the substrate functions as anetching mask. However, while the phase of the diffraction grating can bechanged at will, to construct even one semiconductor laser is extremelytime-consuming; productivity is low, costs are high, and thus the methodis not practical. Alternatively, a positive photoresist, in which areaswhich have been exposed to light are removed by a development process,is coated on the right half of the laser resonator, and a negativephotoresist, in which areas which have not been exposed to light areremoved by a development process, is coated on the left half of theresonator. Then, both the photoresists are exposed by a holographicexposing system and developed, and the remaining photoresist is used asa mask in the succeeding etching process. However, the efficiency of thenegative resist is poor, and the application of the resist is difficult.Moreover, the phase shift is limited to only π.

SUMMARY OF THE INVENTION

The method of this invention which overcomes the above-discussed andnumerous other disadvantages and deficiencies of the prior art, is amethod for the formation of a diffraction grating on a substrate using aholographic technique and an etching technique, wherein the periodicityof the pattern of the diffraction grating can be changed at will by achange of the light-path length of one of the two light fluxes from aholographic exposing system.

The change of the light-path length of one of the two light fluxes fromthe holographic exposing system is, in a preferred embodiment, achievedby passing said light flux through a transparent board of uneventhickness.

The difference Δd in thickness between the thicker portion and thethinner portion of said transparent board has, in a preferredembodiment, the following relationship to the wavelength of the lightpassing through said transparent board:

    n·Δd=λ/4

The width of the thicker portion of said transparent board is, in apreferred embodiment, equal to that of the thinner portion of saidtransparent board.

The width of both the thicker portion and the thinner portion of saidtransparent board is, in a preferred embodiment, equal to the length ofa semiconductor laser resonator which is obtained using the substratehaving said diffraction grating thereon.

Thus, the invention described herein makes possible the objects of (1)providing a simple method for the formation of a diffraction grating,the phase of which is changed by π in the middle of a semiconductorlaser resonator; (2) providing a method for the formation of adiffraction grating which is applied to produce a distributed-feedbacksemiconductor laser device having a diffraction grating the phase ofwhich is changed by π in the middle of the semiconductor laserresonator, said laser device oscillating in a stabilized single modeeven when high-speed modulation is involved as when the laser device isused as a light source for optical communication; and (3) providing amethod for the formation of a diffraction grating which is also appliedto produce a distributed Bragg reflection semiconductor laser device.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects andadvantages will become apparent to those skilled in the art by referenceto the accompanying drawings as follows:

FIG. 1(I) is a front view of the substrate having the diffractiongrating thereon with a uniform phase.

FIG. 1(II) is a front view of the substrate having the diffractiongrating thereon, exhibiting an altered phase pattern in the centerportion of the substrate.

FIG. 2 is a diagram showing a holographic exposing system used in thisinvention.

FIG. 3 is a sectional front view showing the transparent board shown inFIG. 2.

FIG. 4 is a plane view showing interference fringes formed on aphotoresist which is coated on the substrate.

FIGS. 5(I), (II) and (III), respectively, are diagrams showing theproduction processes of a diffraction grating, exhibiting an alteredphase pattern in the center portion of the substrate, according to thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention provides a method for the formation of a diffractiongrating in which by passing one of the two light fluxes from aholographic exposing system through a transparent board of uneventhickness, a change of the light-path length is achieved at will,thereby attaining a change of the periodicity of the pattern of thediffraction grating.

FIG. 2 shows a holographic exposing system used in this invention, whichcomprises a beam splitter 5A and a pair of plane mirrors 5B and 5C. 0nthe light path from the plane mirror 5B to a substrate 6 on which thediffraction grating is formed, a transparent board 7 is disposed.

A light flux 10-1 which is interferential and in parallel, e.g., He-Cdlaser light with a wavelength of 4416Å, is incident upon the beamsplitter 5A of the above-mentioned holographic exposing system to besplit into two light fluxes 10-2 and 10-3 having the same opticalintensity, which are then incident upon the plane mirrors 5B and 5C,respectively, The reflected light fluxes 10-4 and 10-5 from mirrors 5Band 5C, respectively, are superposed on a photoresist coated on thesubstrate 6 resulting in interference fringes. Since the reflected lightflux 10-4 from the mirror 5B reaches the photoresist on the substrate 6through the transparent board 7, the light-path length of the light flux10-4 is different when compared with the other reflected light flux 10-5from the mirror 5C, resulting in interference fringes on the photoresistof the substrate 6.

FIG. 3 shows the transparent board 7 of uneven thickness, in which thewidth L₁ of the thicker portion 7a is equal to the width L₂ of thethinner portion 7b, and the widths L₁ and L₂ are equal to the length Lof the finally obtainable semiconductor laser resonator. Moreover, thedifference Δd in thickness between the thicker portion 7a and thethinner portion 7b of the board 7 (i.e., the difference ΔS between thelight-path length of the light passing through the thicker portion 7aand that of the light passing through the thinner portion 7b) isone-fourth times the wavelength λ of the laser light as represented bythe equation:

    n·Δd=λ/4

(n is a refractive index of the board.)

The transparent board (or a phase board) 7 can be produced by subjectingsilicon oxide film vaporized on a parallel board made of the fusion ofquartz to a photolithographic treatment.

By the use of the transparent board 7 having the above-mentionedstructure, as shown in FIGS. 4 and 5(I), the interference fringes 12which are composed of the light and dark areas with an inversion in thecenter of the middle region A--A' of the substrate 6 are formed on thephotoresist 11 coated on the substrate 6 (the non-slash areas in FIG. 4indicate the photosensitive areas). The photoresist 11 is thendeveloped, and as shown in FIG. 5(II), the photosensitive areas areeliminated, resulting in grooves 13 parallel to each other. Thesubstrate 6 is then etched with an etchant (HBr:HNO₃ :CH₃ OH=1:1:5). Theremaining photoresist serves as an etching mask. The mask is thenremoved, resulting in a diffraction grating 15 on the substrate 6, inwhich the phase of the groove 13 is shifted by π in the center of theobtained semiconductor laser resonator.

Although the above-mentioned example describes only the method for theformation of a diffraction grating the phase of which is changed by π inthe center of a semiconductor laser resonator, it is not limitedthereto. By the modification of the difference Δd in thickness of thetransparent board between the thicker portion and the thinner portionand/or the widths L₁ and L₂ of the thicker portion and the thinnerportion of the transparent board, diffraction gratings having a varietyof phases can be produced.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty which reside inthe present invention, including all features which would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. A method for the formation of a diffractiongrating on a substrate using a holographic technique and an etchingtechnique, the method comprising the steps of:coating a photoresist onsaid substrate; forming interference fringes on said photoresist bysuperposing two light fluxes from a holographic exposing system on saidphotoresist, wherein a transparent board of uneven thickness is placedin the path of one of said two light fluxes; developing said photoresistto eliminate photosensitive areas thereon; etching said substrate usingremaining photoresist as an etching mask; and removing said etching maskfrom said substrate, thus providing a diffraction grating on saidsubstrate, wherein a phase shift is introduced at a discrete pointwithin the diffraction grating by making a difference in light pathlength between one portion and the remaining portion of said one lightflux of said two light fluxes in the holographic exposing system, thedifference in light path length being caused by said transparent boardwherein the difference in light path length of one of the two lightfluxes from the holographic exposing system is achieved by passing saidone light flux through said transparent board of uneven thickness.
 2. Amethod for the formation of a diffraction grating according to claim 1,wherein the difference Δd in thickness between the thicker portion andthe thinner portion of said transparent board has the followingrelationship to the wavelength of the light passing through saidtransparent board:

    n·Δd=μ/4


3. A method for the formation of a diffraction grating according toclaim 2, wherein the width of the thicker portion of said transparentboard is equal to that of the thinner portion of said transparent board.4. A method for the formation of a diffraction grating according toclaim 3, wherein the width of both the thicker portion and the thinnerportion of said transparent board is equal to the length of asemiconductor laser resonator which is obtained using the substratehaving said diffraction grating thereon.